About 40 days from now, Boeing will finish building the first 737 that will lead off the historic production rate of 38 737s a month.

The first step was taken Tuesday when Machinists began working on the wing spar of that first 737 at the Boeing’s Renton plant. The spars are the internal supports for a plane’s wing.

Next year, Boeing will ramp up 737 production to 42 planes a month, yet another record.

“Thirty-eight is a new record for us and next year we go up to 42 and set another record in terms of the the most single-aisle aircraft ever produced on a single production line,” said Beverly Wyse, vice president and general manager of the 737 program.

But it all started when Machinists loaded the spars into an automated machine that assembles the spars.

“The first spar load serves as the defining moment for our latest rate break, and the 737 team did it as planned, on schedule,” said Wyse. “We have more hard work ahead of us, but we are well on our way to another successful production rate increase.”

Already, the 737 is the world’s most popular airplane, with more than 10,000 sold and 4,000 Next Generation 737s delivered.

But coming up fast is the latest variant of the 737, the MAX. Boeing will test fly its first MAX in 2015 and it will enter service in 2017.

Boeing led a media tour Tuesday through the wing-assembly building and the final-assembly building, where 737s get their wings, engines and other essentials.